Live vaccines give rise to better and longer-lasting anti-tuberculous immunity than killed or sub-unit vaccines. Live BCG is the only licensed tuberculosis vaccine in the US but it does little to protect adults against this highly infectious disease. Mice vaccinated with a sublethal inoculum of M. tuberculosis H37Rv develop high levels of protection against an aerogenic challenge infection with M. tb. Erdman given 1 to 3 months later. The avirulent H37Ra vaccine is very safe but induces little or no immunity against this disease. Virulence genes transferred from H37Rv to H37Ra by electoroporation restore the ability of the attenuated recombinant to grow and survive within the spleens of vaccinated animals. The vaccinated mice were challenge 1 - 2 months later with a small aerogenic inoculum of M. tb Erdman and the growth of the organisms in the lungs and the rate of spread to the spleen are compared to that seen in control mice vaccinated with live BCG or the vector control.